Effect of W/Mo ratio on the microstructure of Ni-Mo-W-Cr-Nb based superalloys after thermal exposure

Abstract

The structural materials used in molten salt reactors (MSRs) require enough high-temperature strength, good chemical compatibility with fuels and irradiation damage resistance. The Nb addition and W alloying are two effective optimization paths, which have been proven to respectively improve the embrittlement problems and high-temperature strength of alloys. It is worth exploring whether one full-featured alloy can be developed for the high-temperature MSRs by combining the benefits of Nb addition and W alloying. In this work, four Ni-Mo-W-Cr-Nb based superalloys with different W/Mo ratios were designed by substituting W for Mo in the standard Nb-modified alloy. The microstructural evolution and mechanical properties of these alloys were evaluated systematically after thermal exposure at 750 °C. As the W/Mo ratio increased, more undissolved primary MC carbides remained, while fewer secondary carbides formed in the matrix and at the grain boundaries after thermal exposure. Micro-hardness and tensile tests showed that the substitution of Mo by W can provide an obvious strengthening effect for alloys in the solid-solution treated state. After thermal exposure, all alloys exhibited precipitation-induced strengthening, but the strengthening degree decreased with the W/Mo ratio due to the fewer secondary MC carbides. Based on experimental observations and thermodynamic calculations, it was found that W atoms cannot easily occupy the lattice sites of MC carbides as Mo atoms do, and the absence of secondary carbides in the high-W alloys is due to the undissolved primary MC carbides and the low Mo concentration.